An overview of the structure of the spinal cord, incomplete SCI versus complete SCI, and the most common causes of a spinal cord injury.
Most people are familiar with the basic function of the spinal cord—how it acts as a relay that carries signals from the brain to the rest of the nervous system. This bundle of nerves is protected by the spinal column, which many confuse for the spinal cord itself because of how the two are talked about almost interchangeably.
In fact, when referencing spinal cord injuries, what usually gets referenced is the specific vertebrae of the spinal column where the injury occurred. One reason for this is that since the spinal column is clearly segmented (and the spinal cord resides inside the column), it’s easier to identify injury locations by referencing the vertebrae injured.
Structure of the Spinal Cord
The spinal cord is contained in a sort of cavity in the spine called the “vertebral foramen,” or the spinal canal. This spinal canal is sandwiched between the intervertebral disk/body of the spine at the front and the “spinous process” at the back.
The spinal cord itself has a protective myelin sheath that acts as insulation for individual nerves which increases nerve efficiency so signals from the brain can travel faster. Other portions of the spinal cord include white matter made up of nerve fibers (axons) surrounding a vaguely butterfly-shaped core of grey matter.
The “body” of the vertebra is the large, round portion of bone in the front side of the spine, while the intervertebral disk is the layer of cartilage between each vertebral body that gives the spine a measure of flexibility and support. The spinous process is the backside of the spine that ends in a pointed tip.
A human spinal column is divided into five major sections, with each section having spinal nerves running through multiple vertebrae (with the exception of the coccygeal vertebra, which only has one). These sections are:
- The Cervical Spinal Cord. This section of the spinal cord runs through the first seven vertebrae running from the top of the spine. These sections are typically listed as C1-C7 when being referenced medically. There is an eighth bundle of nerves that exits the spinal column between the C7 vertebra and the Th1 vertebra, however, which is referenced as the C8 nerve, although it doesn’t have a corresponding vertebra. Learn more about the Cervical Spine Cord Injury Levels.
- The Thoracic Spinal Cord. The next twelve vertebrae in the spinal column after the cervical spinal cord ends are called the thoracic spinal cord. This consists of twelve vertebrae that are typically labeled the T1-T12 vertebrae. Learn more about the Thoracic Spinal Cord.
- The Lumbar Spinal Cord. This section of the spinal cord is made of 5 large vertebrae near the base of the spine. These vertebrae are labeled as L1-L5. The end of the spinal cord proper is at the L2 vertebra. After that, it’s actually just nerve roots rather than the spinal cord itself extending from the spine. Learn more about Lumbar Spinal Cord Injuries.
- The Sacral Spinal Nerves. Technically, the sacrum has five separate vertebrae like the lumbar spinal cord. However, these vertebrae have been fused, so they lack the flexibility of other spinal column sections. There are 5 nerve segments in the sacral spinal column, each one labeled S1-S5. The spinal cord itself does not extend into this area of the spinal column, just nerve roots. Learn more about Sacral Spine Injuries.
- The Coccygeal Spinal Nerve. There are two vertebrae in the coccygeal section of the spinal column, but only one spinal nerve bundle between them. Additionally, these vertebrae are often fused in adults.
It is important to note that this is just a very basic, high-level overview of the overall structure of the spinal cord and column. There are many more individual components of each vertebra, as well as differences in size and structure for vertebrae throughout each major region of the spine.
The Effects of Injuries at Different Levels of the Spinal Cord
The effects of an injury to the spinal cord can vary dramatically depending on the location of the injury and its severity. Severity of a spinal cord injury (SCI) is usually categorized as either:
- Incomplete SCI. In these types of injuries, the spinal cord is only partially severed or damaged, which may allow the injured person to retain some function of nerve segments at or below the injury site.
- Complete SCI. Here, the spinal cord is completely severed, eliminating function for nerves below the site of the injury. However, some treatments and therapy regimens have helped SCI survivors with “complete” spinal cord injuries regain some lost function.
The other important component of SCI is the location of the injury along the spinal column. Basically, the closer the injury is to the head, the more function is likely to be lost. For example, an injury of the spine at or above the C3 vertebra may result in complete paralysis to the point the injured person requires a ventilator to breathe, while a C5 injury may leave them some control over their shoulders and biceps.
Those with injuries in the upper thoracic area of the spinal cord (T1-T8) may retain good control over their arms, but not their abdomen. Injuries in the T9-T12 area may allow for good control over the abdomen.
L1-L5 injuries may allow for some limited control over the hips and legs. Injuries to the sacral segment of the spinal cord (S1-S5) could affect control of the groin and extremities in the legs (such as toes).
So, an L-5 incomplete SCI would be much less limiting than a C5 complete SCI—and a C3 or higher injury could prevent signals from passing between the brain and the lungs or heart if severe enough.
What Are the Common Causes of Spinal Cord Injury?
There are many different ways in which a spinal cord injury can occur. The most common causes of SCI based on data from 30,500 spinal cord injuries recorded in 2014 by the National Spinal Cord Injury Statistical Center at the University of Alabama-Birmingham include:
- Auto Accidents. Auto accidents accounted for 10,047 (32.9% of total) SCI injuries in 2014. Here, men were twice as likely to be injured as women (7,205 compared to 2,842).
- Falls (All Types). 6,668 (21.9% of total) spinal injuries occurred as the result of a fall of some kind. Over 4 times as many men (5,406) were injured this way than women (1,262).
- Gunshot Wounds. 4,735 (15.5% of total) spinal injuries were caused by gunshot wounds. Men were almost 4 times as likely as women to be injured this way—4,163 SCI injuries among men to 572 injuries among women.
- Diving Incidents. Diving accidents accounted for 1,840 (6% of total) injuries. In this category, men were more than 10 times as likely to get injured at 1,718 male injuries to 122 female injuries.
All told, these 4 causes account for roughly 76.3% of all spinal cord injuries in 2014. The remaining 23.7% of injuries that year were spread out over 33 other causes. However, this “percentage of injuries” calculation can be misleading, as some inherently dangerous activities might report fewer total injuries since there aren’t as many people performing those actions.
For example, most Americans with steady work or who are attending school are on the roadways daily, either as a driver or a passenger of a motor vehicle. With millions of people on the road every day, even if only a fraction of a percent of them get a spinal injury each year, the number is going to look high compared to the total number of football injuries (153, or 0.5% of all SCIs in 2014) where the activity is performed less frequently and has fewer participants.